The present invention relates to a novel sizing process and more particularly to a method for the internal sizing of paper using black liquor from a pulping process.
The term "sizing" may be defined as the process in which a chemical additive provides paper and paperboard with resistance to liquid wetting, penetration and adsorption. Because cellulose is a very hydrophilic material and pulp fiber surfaces have a high specific energy, water readily wets these surfaces. The very porous nature of paper makes it act like a sponge, so that unsized paper soaks up aqueous liquids very rapidly and extensively. Such adsorption of water produces deleterious losses in paper properties such as strength and stiffness as measured by the Cobb and Hercules sizing tests. Accordingly, it is an accepted practice to add sizing agents to pulp stock during the manufacture of paper to increase water repellency.
Some of the practical requirements for a sizing agent are, (1) it must produce fiber surfaces having a low degree of wettability by aqueous liquids; (2) a feasible method must be available for attaching the size to the fiber surface; (3) the size must be well distributed over all fiber surfaces to insure adequate water repellancy throughout the paper structure; (4) the molecules of the size must eventually be well anchored to the fiber surfaces or become permanently immobilized in some other way; (5) the size must have a high degree of chemical inertness toward the impinging liquid; and, (6) the size should have no adverse effects on either the papermaking process or the paper product properties.
Examples of sizing materials used in the past include natural material such as tall oil rosin, wood rosin and gum rosin. Other sizing agents and processes include wax and asphalt emulsion sizes and various synthetic sizing agents such as alkyl ketene dimer which are chemically reactive. It is also known that tall oil pitch may be used as a sizing agent. However, the present invention suggests the use of black liquor as a sizing agent for paper, and discloses a method for using the black liquor from a chemical pulping process directly in the machine chest of a paper machine for pre-sizing paper.